1. Field of the Invention
This invention relates to forming silicon oxide from poly-crystalline silicon during the fabrication of integrated circuits, and more particularly to the use of silicon nitride as a diffusion stop.
2. Description of the Related Art
During the conventional oxidation step in IC fabrication, silicon material (either single crystal or poly-crystalline) is converted to silicon oxide insulation material by exposure to oxygen at elevated temperatures. Oxygen reacts with the surface silicon forming silicon oxide (oxide). The oxygen diffuses through this oxide to form more oxide. The silicon is progressively oxidized from the surface inward.
An oxide/silicon interface exists between the oxide and the silicon. The interface defines the furthest extent of the oxygen diffusion and oxidation into the interior of the silicon, and therefore is generally oxygen "poor". The limited oxygen condition results in partially oxidized silicon and imperfect oxidation bonds proximate the interface. The interface advances into the silicon as the oxidation step proceeds in the conventional fabrication, and is present in the finished conventional device. The incomplete oxidation results in "loose chemical bonds" which support leakage and breakdown paths during the operation of the device.
The conventional oxidation technique does not employ the present silicon nitride diffusion stop. The extent of the silicon to oxide conversion is therefore time and temperature dependent. If the oxidation step is too short or the temperature too low, the resulting oxide layer is thinner than expected. If the oxidation step is too long or the temperature too high, the resulting oxide layer is thicker than expected.
The conventional oxidation of single crystal silicon material (wafers) has additional problems which are overcome by the present nitride diffusion stop. The dopant concentration within the substance portion of each wafer varies slightly from wafer to wafer. The oxidation rate of the substrate silicon is dependent on this dopant concentration, resulting in a non-uniform oxide thickness from wafer to wafer. Wafers with a higher dopant concentration will undergo more oxide conversion than lower dopant wafers during the same oxidation time period. The substrate oxide will be thicker on these higher dopant wafers. The oxide thickness variation is caused by concentration variances, and is present even if the time and temperature conditions remain constant.
The conventional oxidation of poly-crystalline silicon material (poly) also has additional problems which are overcome by the present nitride diffusion stop. Grain boundaries within the poly accumulate impurities which enhance the local oxidation rate. The resulting oxide is "non-conformal" with the underlying poly. The poly to oxide conversion is greater over each grain boundary because of the enhanced oxidation. The grain boundary regions produce a thicker oxide. The poly to oxide conversion is less over each grain body where the local oxidation rate is not enhanced. The resulting thin oxide regions over the grain bodies are subject to electrical breakdown during operation.
This non-conformal problem is less significant at higher oxidation temperatures (1100 degrees C.). Both grain boundary regions and grain body regions produce oxide at a much faster rate; reducing the boundary oxidation differential. However, at higher temperatures, dopant redistribution by thermal diffusion lowers the performance of the device.
Conventional oxidation techniques for trenches results in "corner stress" in the substrate silicon. The poly expands to over twice its original volume as the oxide is formed. Trench corners have congested geometries which do not permit oxide expansion as readily the flat walls of the trench. The resulting expansion stress is transmitted to the substrate, preceeding the advancing oxide.